This invention relates to an improvement in a commutator apparatus of such rotary electric machine with commutators as a DC motor and a DC generator, or more in particular to an improvement in the commutator apparatus of a rotary electric machine having a comparatively large radial gap between the commutator and the armature coil.
Generally, the commutator of the rotary electric machine of this type mainly comprises a plurality of commutator segments arranged in juxtaposition or side by side on a rotary shaft through an insulator material. Between the commutator segments and the armature winding, there are provided a plurality of risers for electrically connecting them with each other. In some rotary electric machines with the commutator segments and the armature winding thereof in proximity to each other, each end of the armature coils is directly coupled to each of the commutator bars. Generally, however, there is formed some radial gap between the armature coils and the commutator segments, in which gap a plurality of risers are arranged.
Each of the risers is comprised of a conductor and the sole function thereof is to connect electrically the armature winding and the commutator. Since each riser is formed on the rotor, however, it requires a considerable mechanical strength. For this purpose, the riser is generally securely supported on a support ring provided rear each riser. However, since the rotary electric machines have been increased in speed and size, such support means fails to provide a sufficient rigidity against vibrations to the rotational direction, so that the rotary electric machine is likely to vibrate in rotation, sometimes leading to the breakage of the risers, resulting in a serious accident.
Such an accident is especially likely to occur in the case of a commutator apparatus of a large DC rotary electric machine for driving the main roll of a rolling mill, which is subjected to a shock vibration from the roll in operation, i.e., the load. This has been a subject long pending for solution.
A commutator apparatus used for rotary electric machines of high speed and large capacity and comparatively resistant to such vibrations is disclosed, for example, in Japanese Utility Model Publication No. 15201/67. In this apparatus, as shown in FIGS. 7 and 8, risers 3 coupling commutator segments 1 and an armature winding 2 to each other are divided in a plurality of groups and each group is bundled by means of a coupling member 4 thereby to form a large space between the riser groups, so that a path of the cooling air is secured on the one hand and the vibrations of the risers are prevented on the other hand. In the drawing under consideration, reference numeral 5 shows a spacer segment interposed between the armature coils, and numeral 6 an insulation ring for insulating the commutator bar from the rotor shaft.
The commutator riser of this type, however, is not satisfactory in view of the fact that the rigidity thereof against and the ability thereof to attenuate the rotational vibrations are very small for the reasons mentioned below.
From the viewpoint of the strength of materials, each of the risers is an independent curved girder so constructed that adjacent risers do not exert any shearing force or moment on each other by the bending deformation in the rotational direction. Therefore, the bending rigidity of the riser in the rotational direction is very low. Also, a very small bending moment and axial force are exerted between adjacent risers or from the risers to the aramature coil and the commutator bars. As a result, substantially no energy is consumed in these parts for damping the riser vibrations, and therefore the riser vibrations are not substantially deadened by energy consumption.
Assume that this type of riser is used with a DC motor for driving the main roll 9 in tandem rolling of ingots 7 and 8 as shown in FIG. 9. The stress as shown in FIG. 10 is generated in the riser.
Specifically, the riser stress .sigma..sub.a which is generated at the time point t.sub.1 when the biting of the ingot 7 is released by the main roll 9 at the end of the rolling of the ingot 7 is not sufficiently attenuated during the time period .DELTA.t from the time point t.sub.1 to the time point t.sub.2 of biting the next ingot 8. The shock vibration which occurs at this very time point is superimposed on the remaining stress .sigma..sub.b of the riser, thus causing a larger riser stress .sigma..sub.c.
A measurement of this phenomenon will be expressed below in numerals as calculated on the site of an actual rolling mill.
Let the diameter of the main roll D be 1000 mm, the rotational speed of the main roll N be 70 r.p.m., and the gap between adjacent ingots l be 100 mm. Then the ingot speed V is given as ##EQU1## Also the time period .DELTA.t is expressed as EQU .DELTA.t=V/l=3665/100 =0.027 sec
Actual measurements show that the natural frequency of riser vibration f.sub.n is 480 Hz, and the attenuation rate of the riser vibration .tau. is 0.01. Therefore, the riser stress is attenuated to the degree shown below during the time period .DELTA.t. EQU .sigma..sub.b /.sigma.=e.sup.-.tau..omega..sbsp.n.sup..multidot..DELTA.t =e.sup.-.tau.2.pi.f.sbsp.n.sup..multidot..DELTA.t =e.sup. --0.01.times. 2.pi.480.times.0.027.apprxeq.0.5
In other words, the stress of about 50% remains unattenuated in this case, which residual stress increases the riser stress, finally leading to the breakage of the riser.